Preheat the oven to 350°F. Butter an 8×8-inch baking dish or a 9×5 loaf pan.
In a large bowl, whisk the melted butter, brown sugar, and granulated sugar until combined.
Whisk in the eggs and vanilla.
Add the mashed bananas and sour cream. Mix until smooth.
In a separate bowl, whisk the flour, baking soda, salt, cardamom, cinnamon, ginger, allspice, and nutmeg.
Add the dry ingredients to the wet and stir just until no dry streaks remain. Do not overmix. Fold in nuts if using.
Pour into the prepared pan and smooth the top.
Bake 55–65 minutes for a loaf pan, or 35–45 minutes for an 8×8 dish, until the top is golden, the center is set, and a toothpick comes out with a few moist crumbs.
Cool in the pan for 15 minutes, then slice and serve warm or at room temperature.